No, I do not think that rock names imply anything whatsoever about the
facies of the same name. Amphibolites are happy in the granulite facies,
hornblende hornfels and eclogite facies, for instance. One should never
identify a facies based on a hand specimen and indeed they are best used on
large scale maps.
Field terms should be regarded as just that, an attempt at making sense
of a field area. It is always risky if one doesn't cut thin sections of
There is no such thing as "shallow contact metamorphic facies", as
facies simply delineate a portion of P-T space, not the process by which
they formed. There are hornblende hornfels facies and pyroxene hornfels
facies rocks that usually but not always formed during contact
metamorphism. They could be defined by the absence of ferromagnesian
garnet and by assemblages and composition fields that have been lucidly
described by Pattison, for instance. Turner shows a good estimate for the
P-T of these rocks.
>It seems you did not quite get the point I am making. There are rocks that
>have (historically, yes) names, such as amphibolite, blueschist eclogite,
>etc. which imply metamorphism in the respective facies, because the facies
>has been named after them. Does that sound logical? We all know that most
>greenschist facies rocks are neither green nor schists. So what, indeed.
>The best guess for an eclogite is still that it formed in the eclogite
>facies J J J.
>And, apart from the sensible use of facies for large-scale maps (as Dugald
>pointed out), we do ask students about metamorphic facies in outcrops
>where we generally do not carry around microscopes or a microprobe. Are
>your students so clever that they remember the stability limits for every
>mineral assemblage they encounter in the field?
>If yes, maybe you can send us a few.
>You say that "low-pressure facies are also identified by assemblages".
>Fair enough. Can you tell me then where the pressure limit for the
>so-called "shallow contact metamorphic facies" lies, and how you define
>it? If we cannot constrain the facies within at least broad P-T limits,
>they may just as well be dumped.
>School of Geological & Computer Sciences
>University of Natal
Professor of Geology
Department of Geological Sciences
2534 C.C. Little Bldg.
425 E. University Ave.
University of Michigan
Ann Arbor MI 48109-1063 USA